A battery is a device that includes one or more electrochemical cells, and converts stored chemical energy into electrical energy. In some important applications, one or more batteries are used to store a portion of the energy in mobile systems such as electric vehicles and hybrid electric vehicles. Examples include locomotives, off-highway mining vehicles, marine applications, buses, and automobiles. The batteries can also be used in important stationary applications, such as uninterruptible power supply (UPS) systems and diesel-battery hybrid systems for off-grid or weak-grid telecommunication stations. High-temperature sodium-metal halide electrochemical cells are often targeted for many of these applications.
A sodium-metal halide battery is typically made up of many sodium-metal halide cells. The cell usually includes a negative electrode comprising sodium; a positive electrode comprising nickel chloride; a molten salt electrolyte comprising sodium tetrachloroaluminate; and a solid electrolyte partitioning the positive electrode from the negative electrode. One surface of the solid electrolyte is in contact with the positive electrode, and another surface is in contact with the negative electrode. Multiple sodium metal halide cells can be connected in series or parallel to form a stack.
Critical end use applications like UPS require the batteries to be in standby mode for most of their life, but also require them to provide high power (e.g., 120 W-250 W per cell) when called for, for time periods ranging from about 30 seconds to 15 minutes. These requirements position sodium metal-halide batteries as good candidates for demanding applications, in terms of high power density and relatively low cost. The batteries are often expected to operate in this manner over a life span of up to about 20 years.
In standby mode, batteries used in applications like UPS often need to be connected directly to the critical electrical load, so that they can support the load immediately in case of a “mains” power failure. This requires the batteries to be kept at a charging voltage. This state is referred to as float charging, or simply, “float”. Techniques for float charging are usually successful at off-setting self-discharge reactions, and maintaining the batteries in a fully-charged state.
However, sodium metal-halide batteries usually do not experience self-discharge, and can sometimes exhibit an increase in electrical resistance during float charging. This increase is undesirable, because it can lead to lower power delivery and/or support time from the battery, i.e., below specified requirements for critical applications, including those that may be situated in remote locations. The resistance rise often appears to be directly related to the applied float-charge voltage. Moreover, the resistance can often constitute two different phenomena. One is an early resistance-rise that usually occurs over the initial 0-2 ampere-hours discharged, referred to as a “low amp-hour” resistance rise. The second event is a more constant, uniform (“series”) resistance-rise that is generally present through most or all of the discharge. Either of these resistance-events can adversely affect the performance of the battery.
In view of some of these concerns and challenges, new methods for maintaining the performance level of a sodium-metal halide electrochemical cell or other type of energy storage device would be welcome in the art. The new techniques should lower the resistance during various times in a cell's time-on-float, thereby restoring the cell—at least on a partial basis—to its original power capability, and improving its projected lifespan. Moreover the techniques should be relatively straightforward and economical to implement.